Various systems for communication via a base station have been developed as a communication system for a radio communication. Specifically, in the case of a communication system which has already been put into practice as a radio telephone, for example, a telephone circuit for a radio communication is set between a base station connected through a wire telephone circuit to a telephone exchange side and a radio telephone apparatus (a mobile station) which is a terminal side telephone, thereby a call service being carried out between a called telephone connected through the telephone exchange and the radio telephone apparatus.
Such radio telephone apparatus as a terminal side apparatus includes a miniaturized portable radio telephone and a land mobile radio telephone mounted on an automobile, etc.
In a communication system employing such radio telephone apparatus, communication between the base station and the terminal side telephone apparatus has been carried out by a radio transmission of an analog signal. On the other hand, in order to efficiently use a transmission band, a communication system for carrying out communication between the base station and the terminal side by a radio transmission of a digital data is developed.
In this digital communication system, transmission and reception of the digital data between the base station and the mobile station are carried out in accordance with an efficient transmission system such as a TDM system (time division multiplex system), a TDMA system (time division multiple access system) and so on, and an audio signal is subjected to a high-efficiency coding and decoding. Therefore, as compared with an analog communication system, it is possible to use a transmission line very efficiently for thereby setting more transmission channels and so on and to realize transmission of a call sound having less noise.
Another communication system to which the digital communication system is applied includes a CDMA system (code division multiple access system), an FDMA system (frequency division multiple access system) and so on. In these systems, the transmission or the like is more efficient as compared with communication carried out by analog transmission.
An example of a TDMA system communication system which is proposed at present will be described. Frequency bands of 800 MHz and 1.5 GHz are used for communication, and in each of the frequency bands, a channel interval is set to 25 kHz. Transmission in a down circuit from the base station to the mobile station is carried out by the TDM system, and transmission in an up circuit from the mobile station to the base station is carried out by the TDMA system. A phase modulation such as a .pi./4 shift DQPSK modulation, for example, is employed as a transmission data modulation system.
In the transmission of the TDM system and the TDMA system, as shown in FIG. 1, for example, assuming that one frame is formed of six time slots of time slots 0 to 5 and a sub frame is formed of a half of one frame, transmission in the down circuit from the base station to the mobile station, for example, is carried out at timings of the time slots 1 and 4 of the sub frame period, and transmission in the up circuit from the mobile station to the base station is carried out during predetermined periods immediately before the time slots for transmission in the down circuit. In this case, the same frequency is used for transmission in each time slot.
A signal transmitted from the base station has a synchronizing word whose bit number is about 20 bits and which is located at a substantial center thereof. The synchronizing word is previously determined depending on systems, and hence the same synchronizing word is used in each of the base stations of the same radio telephone system.
The mobile station side receives the synchronizing word transmitted from the base station and carries out a synchronizing processing for communication with the base station with reference to a timing at which it receives the synchronizing word.
Each of the base stations transmits a broadcasting information on a predetermined frequency channel and time slot. When each of the mobile stations carries out an initial reception (i.e., when it initially searches for a base station with which it can communicate), it carries out the reception processing with respect to all the channels and time slots on which the broadcasting information may be transmitted. It measures a received electric field intensity (RSSI) and so on with respect to the respective channels and time slots to determine that a signal on a channel on which the broadcasting information can precisely be received with a high received electric field intensity is a signal from the closest base station. Then, each of the mobile stations synchronizes its reception frequency with that of the signal on the determined channel.
After commencing communication with a certain base station as described above, each of the mobile stations receives a signal on a channel of a cell adjacent to the base station during an idle slot (i.e., a period during which it does not communicate with the base station) based on information of a frequency channel of the adjacent cell which is transmitted from the base station in the broadcasting information, measuring its received electric field intensity. When the received electric field intensity of the adjacent cell becomes more higher as compared with the received electric field intensity of the cell in which it is communicating with the base unit, the mobile station carries out a hand-off processing for switching communication to communication with a base station of the adjacent cell.
Each of the base stations has a defined synchronizing burst. When the hand-off processing is carried out, the synchronizing burst is transmitted and received between the base station and the mobile station, thereby the mobile station obtaining synchronization with the base station.
In the case of the digital radio telephone system in which the transmission is carried out in accordance with such TDMA system, while multiple access on one channel is made possible by independently using time slots prepared in one frame and the transmission channel can be used efficiently, a ratio in time used for communication with each mobile station is fixed regardless of a communication state. Specifically, in the case of the example shown in FIG. 1, for example, since two time slots of six time slots prepared in one frame are used for communication, the time ratio is fixed to 1/3. When the time ratio is thus fixed, it is easy to control communication, while it is sometimes possible to carry out communication with less time ratio depending upon a state of communication between the base station and the mobile station, and when the communication state is not satisfactory, communication with longer time ratio is sometimes requested. However, the conventional communication system of this kind has a fixed time ratio regardless of the communication state, and hence the circuit is not always used efficiently.
Since transmission of the TDMA system has been described above, the time ratio is fixed. In the case of the CDMA system, the communication is carried out with a fixed use ratio of codes. In the case of the FDMA system, the communication is carried out with a fixed use ratio of a frequency.
One frequency is allocated to one circuit. When the mobile station is in such a slow fading state that it is comparatively slowly moving due to a user's walking or the like, once the mobile station is brought in a bottom of a fading state where a reception state is unsatisfactory, it takes long time for the mobile station to be brought out of the bottom of the fading state. Therefore, an unsatisfactory communication state is continued comparatively long and hence a speech quality is deteriorated temporarily.
The synchronizing word allocated to each time slot is highly likely to be erroneously detected because of mutual interference with another data in the same time slot. Therefore, it is necessary to take some countermeasure for preventing an erroneous synchronization.
Since the synchronizing word is formed of a comparatively small number of bits, about 20 bits, it is difficult to estimate a state of a transmission line based on a reception state of the synchronizing word.
When each of the mobile stations carries out the initial reception, it is necessary for it to carry out a processing for searching a channel on which a signal can satisfactorily be detected, by receiving signals on all the channels and time slots on which the broadcasting information may be transmitted. Therefore, there is then the disadvantage that it takes long time for the mobile station to detect the channel on which communication can be carried out.
When the mobile station carries out the hand-off processing for switching the communication to the communication with the adjacent cell, it is necessary for it to receive information of the channel frequency of the adjacent base station from the base station. Therefore, each of the base stations is required to store information of allocated frequencies of all the stations adjacent thereto and so on. Moreover, each of the mobile stations needs to receive control signals from the adjacent base stations to measure their electric field intensities during a period of the idle time slot on which the communication with the base station is not carried out. Therefore, since it is required to carry out reception which does not relates directly to communication for telephone call, a reception time required at the mobile station is increased to that extent. A power required for the communication is increased to that extent and an arrangement for control thereof is required, which complicates an arrangement of an apparatus of the mobile station.
When the mobile station carries out the hand-off processing, it is necessary to receive the synchronizing burst of a station to which the base station is switched for obtaining the synchronization. Upon the hand-off processing, sometimes there may temporarily be a time during which the mobile station cannot communicate with the station to which the base station is switched until the synchronization with the station is obtained. Once this accident happens, the call sound is temporarily interrupted upon the hand-off processing.